Terminal units of the type referred to typically comprise a duct through which conditioned air flows from a supply to the space to be conditioned, and a diverter blade within the duct that can be positioned to vary the volume of air entering the space by diverting some of the air to a by-pass outlet in the duct. The air that is diverted by-passes the space and may, for example, flow into a return air plenum of the overall air handling system. Units of this type are often referred to as "by-pass boxes".
The duct is usually of rectangular shape in cross-section and has an inlet at one end and an outlet at the other end so that the conditioned air normally flows straight through the duct. The by-pass outlet is provided by an opening in the wall of the duct, for example at the top, and the blade is positioned within the duct so that it can deflect all or some of the conditioned air to the by-pass outlet.
In its simplest form, the blade takes the form of a plain rectangular plate that is pivoted to the wall of the duct adjacent the by-pass outlet opening, for movement between a position in which it closes the opening so that all of the air flows straight through the duct, and a position in which the blade is angled with respect to the incoming air so that the air is diverted by the blade into the by-pass opening. Typically, the blade is mounted on a shaft that is pivoted to the wall of the duct and an actuator is mounted externally on the duct and coupled to the shaft for turning the shaft between its two extreme positions under the control of a thermostat in the space to be conditioned.
A disadvantage of this diverter blade arrangement is that air impinging against the blade when the blade is in an angled position across the duct imposes relatively high torque forces on the shaft that must be overcome by the actuator when the blade is to be moved. Relatively powerful actuators must therefore be used and the service life of the actuator is often detrimentally affected.
In an attempt to address these problems, it has been proposed to mount the blade on a pivot shaft that is located at the center of the blade and extends transversely of the duct at the mid-point of its height. In this configuration, part of the blade is above the pivot shaft and part below so that the turning effect on the shaft imposed by the air impinging on one part of the blade is counter-acted and ideally balanced by the air that impinges on the other part of the blade.
While this blade arrangement avoids the imposition of high torque loads on the blade pivot shaft, an auxiliary blade must be provided to close the by-pass outlet in the duct when all of the air is to flow straight through. Normally, the auxiliary blade is pivoted to the duct at the side of the by-pass opening but out of the main air flow through the duct, and the auxiliary blade is coupled to the main diverter blade by a linkage so that the auxiliary blade is opened and closed automatically in response to turning of the main diverter blade under the control of the actuator.
This arrangement not only introduces additional components and, therefore, cost and attendant service difficulties, but the actuator for the main diverter blade must now be powerful enough to move both blades.